The present invention relates to a data storage system, and more particularly to the detection of multilevel data in a system having a shift register storage device.
Semiconductor charge-transfer devices, such as charge coupled devices (CCD's), are well known and have been used and proposed for use in various applications relating to the storage of data.
One problem associated with CCD's is the difficulty in reliably and accurately detecting the data stored in the device. A CCD operates much like a serial shift register, with the bits of information represented by charge packets serially shifted through the CCD. During the shifting, the CCD frequently experiences charge losses. The losses are affected by various considerations, and can vary from one device to another. For example, CCD's are sensitive to variations in temperature, clocking frequency and the physical dimensions within the device.
In the past, detection of data stored within a CCD has been accomplished by comparing the signal from each cell with an external reference signal. However, if significant cell-to-cell losses occur, such a comparison does not reliably detect the stored data. To minimize cell-to-cell losses, it is necessary that there be frequent regeneration of the data and that operating temperatures be kept within specified ranges, among other things. These constraints have increased the cost and physical size of CCD's and thus made them undesirable for many memory applications.
This problem is particularly acute in a multilevel, non-binary system where the signal from each cell represents one of three or more data levels. Unpredictable cell-to-cell losses make it very difficult to distinguish between several possible data levels by comparing the signal from each cell to an external reference signal. Even frequent regeneration and maintaining strict operating conditions has not made multilevel, non-binary data storage in CCD's feasible.
In co-pending U.S. application Ser. No. 930,613, entitled "DATA STORAGE SYSTEM," by William P. Ward, filed Aug. 3, 1978, there is disclosed a data storage system having a CCD shift register for storing binary data, with the data detected by comparing signals in adjacent storage cells of the CCD shift register. However, application Ser. No. 930,613 does not provide for a non-binary system, where three or more data signal levels may be stored in each storage cell.